1. Technical Field of the Invention
The present invention relates generally to a spark plug which may be employed in automotive vehicles, gas pumps, and cogeneration systems, and more particularly to a spark plug which is so designed as to provide high thermal resistance and durability without sacrificing the ignitability of gaseous fuel.
2. Background Art
Typical spark plugs have a center electrode installed in a metal shell through an electric insulator and a ground electrode joined to the metal shell. The ground electrode faces the tip of the center electrode through an air gap (also called a spark gap) in which a sequence of sparks are produced.
In recent years, spark plugs are used which have a noble metal chip welded to an end of the ground electrode exposed to the spark gap in order to prolong the service life and improve the performance of the spark plug. The noble metal chip is made from, for example, Pt (Platinum) or Ir (Iridium) which is excellent in wear resistance. This type of spark plug, however, encounters a drawback in that a fused portion that is a weld between the noble metal chip and the ground electrode exists near the spark gap, so that it is subjected to intense heat within a combustion chamber of the engine, thereby resulting in great wear of the fused portion. In the worst case, it results in dislodgement of the noble metal chip from the ground electrode and an increase in spark gap.
It is therefore a principal object of the invention to avoid the disadvantages of the prior art.
It is another object of the invention to provide a spark plug designed to provide higher thermal resistance and durability without sacrificing the ignitability of a gaseous fuel.
According to one aspect of the invention, there is provided a spark plug which may be employed in automotive vehicles, gas pumps, and cogeneration systems and which is so designed as to provide higher durability and productivity. The spark plug comprises: (a) a metal shell; (b) a center electrode retained within the metal shell to be insulated from the metal shell, the center electrode having a given length and a tip projecting from the metal shell; and (d) a ground electrode including a noble metal-made tip and a body, the body being joined to the metal shell outside the center electrode in a lateral direction of the spark plug. The noble metal-made tip is connected to the body through a fused portion formed by materials of the body and the noble metal-made tip melted together and extends toward the center electrode to define a spark gap between the noble metal-made tip and the center electrode. If a portion of the noble metal-made tip of the ground electrode closest to the tip of the center electrode is defined as a ground electrode tip, and a portion of the fused portion closest to the tip of the center electrode is defined as a fused portion tip, the ground electrode tip and the fused portion tip are located within a range defined by a first line extending from the tip of the center electrode in a lateral direction of the center electrode and a second line extending from a portion of the center electrode closest to the ground electrode in a longitudinal direction parallel to a longitudinal center line of the center electrode so that the ground electrode tip and the fused portion tip do not overlap with the tip of the center electrode both in the lateral direction and in the longitudinal direction of the center electrode, thereby securing the stability of a sequence of sparks without sacrificing the growth of a flame kernel, which results in improved ignitability of fuel. The fused portion is not located on a line passing through the spark gap, thus avoiding the dislodgement of the noble metal-made tip from the body of the ground electrode due to spark-caused wear of the fused portion. The fused portion has a positional relation to the center electrode so that it does not overlap with the center electrode in a radial and vertical directions of the center electrode, thus avoiding the possibility of sparks occurring between a side wall of the center electrode and the fused portion, which minimizes the wear of the fused portion. This results in improved thermal resistance and durability of the ground electrode, which prolongs the service life of the spark plug.
In the preferred mode of the invention, a minimum distance between the fused portion tip and the tip of the center electrode is greater than the spark gap by 0.2 mm or more.
The noble metal-made tip of the ground electrode is made of a cylindrical member which has a diameter of 0.4 mm to 0.8 mm.
The noble metal-made tip of the ground electrode has a length projecting the fused portion which is between 0.3 mm and 1.0 mm.
The body of the ground electrode has a first portion and a second portion. The first portion is joined to same metal shell. The second portion extends from the first portion toward the longitudinal center line of the center electrode. A longitudinal center line of the second portion extends parallel to a longitudinal center line of the noble metal-made tip of the ground electrode.
The noble metal-made tip of the ground electrode is made of one of an Ir alloy and a Pt alloy.